FIG. 7 is a sectional view showing a structure of a conventional electronically controlled throttle body. The throttle body 1 has a circular bore 2a at the center of a main body 2, and a circular-disc-shaped throttle valve 3 is disposed therein. The throttle valve 3 is fixed with two screws 5, 5 to a throttle shaft 4 which pierces the bore 2a, and is free to rotate from the position to close the bore 2a to a full-open position being parallel to the center axis of the bore 2a. The rotating range is 90 degrees at the maximum, and no more range is needed.
A motor 6 is integrally attached to the throttle body 1, and the shaft of the motor 6 is integral with the throttle shaft 4. Here, by changing the power supply direction, the throttle shaft 4 turns in the opening direction or the closing direction.
A torque motor is adopted as the motor 6. In general, a torque motor has characteristics of having excellent responsiveness and high reliability since there is no contact. The motor 6 of this kind generally has a rotor to which a ring-shaped magnet is fixed, and controls a rotating position in accordance with changes of magnetic flux distribution formed by a coil and a magnetic path.
As mentioned above, with the throttle body 1, the rotating range of the throttle valve to open and close the bore 2a is 90 degrees at the maximum. For example, when an inclination of about 5 degrees is set at idling, the rotating range becomes about 85 degrees. Consequently, the rotating range of the throttle valve 3 is 90 degrees or less. To drive and control within this range, the magnet is not needed over the whole circumference. In addition, the magnet used for the rotor is expensive since the magnetic flux density has to be high.
Therefore, a torque motor 10 utilizing segment type magnets was devised, as shown in FIG. 8. A rotor 11 of this figure is connected directly to the throttle shaft 4 in FIG. 7. About two thirds of the circumference of the rotor 11 is covered by two segment type magnets 12, 12. Since the magnet is downsized by changing from a ring-shape to a segment type, the cost can be reduced. An air-gap is formed between the circumference face of the magnets 12, 12 and a yoke 13. Another air-gap is formed between the circumference face of the magnets 12, 12 and a core 14. A coil 15 is disposed at the core 14.
Parts of the yoke 13 corresponding to the magnet 12, 12 are first and second magnetic sides 13a, 13b whose top end faces are arc-shaped. Similarly, a part of the core 14 corresponding to the magnet 12, 12 is a third magnetic side 14a whose top end face is arc-shaped. Then, these three magnetic sides 13a, 13b, 14a are located on the same arc. Further, a stator is constructed by the yoke 13, the core 14 and the coil 15, and a moving portion is constructed by the rotor 11 and the magnets 12, 12.
When electric current is supplied to the coil 15, the rotor 11 rotates around the axis O, and the throttle valve 3 which is directly connected to the rotor 11 opens and closes. The rotating direction of the rotor 11 changes in accordance with the direction of the electric current which passes through the coil 15. With the above-mentioned torque motor 10, the rotating angle of the rotor 11 is about 120 degrees, because the magnets 12, 12 cover about two thirds of the circumference of the rotor 11.
However, as mentioned above, since the rotating angle of the throttle shaft 4 is approximately between 85 degrees and 90 degrees, the use of the torque motor 10 in FIG. 8 is not efficient.
Further, the above-mentioned torque motor 10 has a characteristic that the torque generated at both ends is lower than the torque generated at the center of the rotating range. This seems to be caused by magnetic circuit problems, such as the magnetizing angle of the magnet, the magnetic saturation of the magnetic poles, and so on. On the contrary, in a normal usage situation, the throttle valve 3 is operated with approximately equal torque from the full-close position to the full-open position. Therefore, it is desirable to obtain a flat torque characteristic. Further, considering freezing in the winter, it is more desirable that the torque at the full-close position is the maximum torque.
For efficiency, adopting a speed reduction mechanism which transmits the rotating angle of the rotor 11 to the throttle shaft 4 via a speed reducer to reduce the angle has been considered. However, having a separate speed reducing mechanism is not desirable since it increases the size of the throttle body. Further, the cost increases because the number of parts increases.
On the other hand, as shown in FIG. 9, adopting a linear type torque motor 20 to obtain a flat torque characteristic has been considered. The torque motor 20 is disclosed in patent application No. 2000-4107 which was previously submitted by the same applicant as this application. The torque motor 20 shown in FIG. 9 has a first stator 21 shaped almost like a rectangle, a second stator 22 shaped like three sides of a shallow rectangle which is disposed with a gap 23 to the first stator 21, an electromagnetic coil 24 which is disposed between the first stator 21 and the second stator 22, a slider 25, and two magnetized members 26, 27 which are attached to the slider 25. The magnetized members 26, 27 are plate-shaped magnets which have magnetic poles in the thickness direction (the vertical direction in FIG. 9), and disposed so that the magnetic polarities of the magnetized members 26, 27 which are adjacent to each other are opposite to each other.
The first stator 21 has two magnetic sides 21a, 21b, and the second stator 22 has one magnetic side 22a. These three magnetic sides are located in a line, and a gap 28 is maintained between the magnetized members 26, 27 of the slider 25 and the magnetic sides 21a, 21b, 22a. 
With the linear type torque motor 20, a stator is structured by the first stator 21, the second stator 22 and the electromagnetic coil 24, and a moving portion is structured by the slider 25 and magnetized members 26, 27. Then, in accordance with the direction of electric current to the electromagnetic coil 24, the slider moves in both directions shown by the arrow.
Here, the actuating force applied to the slider 25 is almost constant no matter where the slider 25 is positioned. Therefore, by transmitting the movement of the slider 25 to the throttle shaft 4, the rotating torque which is applied to the throttle shaft 4 can be almost constant.
However, to convert linear motion of the slider 25 to rotating motion of the throttle shaft 4, separate parts are needed and the structure is complicated.
The present invention is devised in consideration of the above-mentioned facts, and the object is to provide an electronically controlled throttle body which can efficiently transmit the motion of a torque motor, including a linear torque motor, to a throttle shaft with a simple structure.